Field of the Invention
The present invention relates to a method for controlling an internal combustion engine as it enters low-idle speed and, more particularly, to such a method designed for an engine associated with means for slaving its speed to a predetermined reference point value.
FIG. 1 of the appended drawing represents an internal combustion engine associated with such means. The engine 1 comprises an air inlet manifold 2 connected to a filter 3 and to an exhaust pipe 4, in which there is conventionally placed a muffler 5, possibly a catalytic converter. In a motor vehicle propelled by such an engine, there is also commonly found nowadays an electronic computer 6 fed by lines 7 for transmitting signals emitted by sensors (not represented) for sensing the speed of the engine, the air inlet pressure, the cooling water temperature, etc. . . . In return, the computer formulates, as a function of various strategies, orders for controlling actuators such as a spark plug 8 for igniting an air/fuel mixture, a fuel injector 9, a butterfly valve 10 for regulating the amount of air let in and a valve 11 controlling an additional air flow, mounted in parallel with the butterfly valve 10. At low-idle speed, with the air butterfly valve 10 closed, the computer regulates the "filling" of the cylinders of the engine with air, with the aid of the additional valve 11, so that the engine therefore turns over at a speed typically of the order of approximately 700 rpm.
As low-idle speed is entered, problems in matching the output torque of the engine to the resistive torques opposing it may be encountered. A "free" drop in the speed of the engine toward a fixed final reference point, as represented in FIG. 2 of the appended drawing, may cause this speed N to drop below the reference low-idle speed N (after the moment t) if friction internal to the engine and due, for example, to cold-starting or auxiliary equipment (alternator, compressor for climate control, power-assisted steering, etc.) increases the load on the engine. The engine may then sputter or even stall.
In order to overcome this drawback, European Patent No. 0 170 574 discloses a method, illustrated in FIG. 3, according to which, as low-idle speed is entered, the final fixed reference point N is replaced by a variable reference point N making it possible to cause the speed of the engine to change smoothly toward this final reference point. Solid line and broken line have been used respectively to represent the change in the actual speed N and in the reference speed N. The latter is fixed, at each moment t, to the value of the low-idle speed N increased by a fraction (100-x) % of the difference between the actual speed N and the low-idle speed N and thus progressively comes to meet this final low-idle speed. Regulators or "controllers" of the PI or PID type act, for example, on the degree of opening of the additional air control valve 11 in order to ensure that the reference point thus fixed is followed.
The use of such controllers is not without drawbacks. In point of fact, while the actual speed is dropping toward the final low-idle speed value, the intermediate reference point N is always below the actual speed N and there is therefore constant incrementation of the integral term in the regulation, which may give rise to a compensating underspeed after the actual speed has reached the final low-idle speed. Moreover, as the reference speed is a function of the actual speed, disturbances (surging) which may affect the latter have a knock-on effect on the reference point, with the introduction of phase shifts by the differential term of the regulation.
Furthermore, Patent Application GB 2 162 973 discloses a method for entering low-idle regulation which is characterized by the following of a reference point of exponential overall form, but the parameters of which depend in particular on the derivative of the speed as a speed threshold is crossed. Such a method exhibits drawbacks connected with the dependence of the reference point curve with respect to the initial conditions. Mention may be made, for example, of the case where the threshold for entering into low-idle regulation is crossed during abrupt braking of the vehicle, with the motor engaged. In this case, the derivative of the speed is high, leading to a very flat reference point curve. As soon as the engine is disengaged, the speed tends to catch up to the reference point curve, then situated very much above it, and this results in abrupt reacceleration of the engine. In the opposite case, very slow deceleration, the reference point curve is very steep, and in this case the risk is that engaging a load such as climate control, etc., may lead to the engine stalling.